Control for measuring air intake for internal combustion engines such as gasoline engines using an air-flow sensor provided within an air inlet pipe, and for injecting fuel in accordance with the quantity of air drawn in by the engine is carried out. Conventionally, a platinum element has been used as the air-flow sensor to measure air intake to an engine in this manner accurately. Most recently, air-flow sensors are essential for air-flow control in air conditioning equipment, and control of wind speed and air flow etc. in environmental equipment.
There are two types of air-flow sensors: a hot type (also called a hot wire type) that utilizes change in resistance of a heated wire as a result of current flowing through a platinum element (platinum hot wire) so as to increase the temperature through self-heating, and losing heat as the air hits its heating portion; and a cold type that measures air temperature as is. Since either type of sensor needs to be placed within an air flow and to measure the temperature of the air accurately, it is necessary to control fluctuation in element resistance, fluctuation in TCR (temperature coefficient of resistance), fluctuation in how heat dissipates, and fluctuation due to turbulence of air flow around the element, etc.
While platinum that has stable properties and excellent resistance-temperature characteristics has been used as a temperature-sensitive material for the element, reduction in element size and heat capacity is necessary in order to quickly respond to change in input (temperature change). In particular, since the hot type element mentioned above utilizes change in resistance, there is a pressing demand against fluctuation in characteristics leading directly to error of measurement.
On the other hand, there are two types of temperature sensors broadly when classified according to configuration: a planar element and a winding-type axial round bar element. Patent Document 1, for example, discloses a winding-type element, such as a resistor element having a platinum wire as a resistor that is wound around a peripheral surface of a ceramic pipe and utilizing temperature dependency on resistance of the resistor.
Patent Document 2, for example, discloses a planar element, such as a thin-film thermistor formed by layering a first thermosensitive film and a second thermosensitive film made of a compound oxide, etc. on a ceramic substrate and providing an insulating layer between the ceramic substrate and the first thermosensitive film, thereby preventing degradation of thermal responsiveness and resistance-temperature characteristics. Moreover, Patent Document 3 discloses a temperature sensor where a substrate and a resistance-meandering part are covered by an intermediate layer (insulating layer) so as to stably hold the resistance-meandering part even if the temperature changes rapidly.
Patent Documents (as referenced herein):    Patent Document 1: JP H3-268302A    Patent Document 2: JP H6-61012A    Patent Document 3: JP 2015-532438A